Sabara is the historical city nearest to Belo Horizonte, so if you do not have much time at BH, it is a good choice having a day tour at Sabara. From BH's bus station you can take a cheap bus and arrive in Sabara's downtown in about half hour. Walking around downtown, you will find Churches built by barroco's architecture as well the whole historical center. In front of the main square, there is a typical food restaurant, which is not so expensive. Its name is 314 restaurant, you cannot leave the town without having lunch there, it is quite tasty.
PS.: You can get touristic information at a tour center right on the historical center entrance. Ask the bus driver to drop you there.